Ceiling-mounted bicycle lift
From DDL Wiki
This article was contributed as part of a design project for the course 24-441 Engineering Design in the spring semester of 2007 at Carnegie Mellon University.
Team 1 - Report 2: Ceiling-mounted bicycle lift redesign
Team 1 - Report 3: Ceiling-mounted storage lift
Contents |
Current Product Analysis
Executive Summary
The ceiling-mounted bicycle lift provides a means to store a single bicycle in a commonly unused space near the ceiling. The pulleys provide a mechanical advantage to make lifting a bicycle much easier. The product is made of durable components and assembles easily to fit a variety of different sized bicycles. The provided directions were unclear and confusing, but assembling the product using general intuition proved not to be a problem. Using the product is simple for the physically active user. By dissecting our product and analyzing each component, we were able to understand the design decisions that were made for the manufacture and assembly of the product. Utilization of duplicate and standard parts and keeping the total part count low is an excellent example of the design intent that went into reducing the manufacturing cost of the ceiling-mounted bicycle lift. The minimization of total parts and use of duplicate parts also aid in assembly. In addition, the incorporation of distinct sub-assemblies creates a more modular design adding to the overall ease of assembling the product by the user. A detailed FMEA shows that the biggests risks are posed by the user themself and the loosening of fasteners due to use over time. The best ways to reduce these risks are to emphasize proper use of the product and maintenance and tightening of fasteners in the user manual. The best improvements to the current product can generally be made in the user manual. Proper instructions for assembly and proper emphasis in maintenance of the product would aid the user to assemble the product with ease and help prevent failures.
Customer Needs and Product Requirements
The Bike Lift has several interesting features that empower the user to store their bike safely and neatly. First, the lift provides the customer with a designated spot where they can always store their bike. The customer never needs to figure out where to put the bike to keep it out of everyone’s way; the lift establishes a consistent, isolated location for storage. This is desirable for the user because it is one less thing they have to worry about after an enjoyable ride.
The safety and ease of use features are pleasing for the consumer. The pulley mounts are secured dependably into the joists of the ceiling or into an unfailing 2 by 4. These mounts house the rope and pulleys which provide the mechanical advantage for lifting the bike. Customers love this aspect of the lift because it is not straining to lift the bike well out of harms way. This product meets the customer’s requests because it provides an all-purpose solution to enable users to store their bike in a protected, organized, and out of sight manner.
The product does not have a multitude of complex parts which require heavy analysis, which is convenient. In order to satisfy accurate operation, the product must be comprised of strong, reliable materials. The screws, mounts, pulleys and hooks must be made of trustworthy materials. Also, the product must be installed correctly, which implies that the installation directions must be very clear and concise. The rope must be of sufficient length so that each customer can operate the lift under their specific storage desires (high ceilings, large bikes, etc.). Lastly, the product must be extremely durable. A user may raise many different bikes, many different times, so the product must be reliable and last for a considerable amount of time pleasing each and every customer beyond their expectations.
Product Function
After purchase, the first step in using the Ceiling Mounted Bicycle Lift is to install the system in the desired location. A set of instructions is included with the product. However, after reviewing the instructions, we determined that they contained many errors and may confuse the user if they were to follow the directions exactly. Therefore, we have developed the following setup procedure to describe the way a generally knowledgeable user may install it without using the given instructions. The Ceiling-mounted bicycle lift setup wiki page shows how we set up the bike lift.
The bike lift was fairly easy to use. The four athletic mechanical engineers in our group did not run into any problems when lifting and lowering the bike. We also had other, not as mechanically inclined, young athletic people see if they could lift and lower the bike without problems. They were able to. We did not have any older, less athletic people try the lift. We would imagine that they may have problems using this lift. However, since the typical user of this product will be physically able to ride a bicycle, we do not think that product will pose any problems for its intended users. The Ceiling-mounted bicycle lift use wiki page shows a detailed account of the proper use of the bike lift.
Parts List
Our Ceiling-mounted bicycle lift parts list contains the quantity of each component in our product, the weight of that component, its function and its most likely manufacturing process. It also contains both a CAD image and a digital picture of each component. The total number of parts in the product is 66 with 19 different parts. The total weight is 1.595 kilograms (3.51 lbs). The majority of parts are made of steel and manufactured by stamping and bending processes. The ceiling mounted bike lift has been designed to use a minimal number of parts and those parts are individually designed to be easily manufactured and usable in other products. The DFMA section of this report goes into further detail of our thoughts on the design.
Structural Analysis
Summary
To determine the overall load capacity of the product we decided to perform a structural analysis on the components that had any chance of being the weakest link of the assembly. Out of the 19 parts, we were able to rule out several parts, such as the clutch components of the locking mechanism (9,10) and the bolts, nuts and washers which have an obvious robust design. The results from the remaining components yielded a minimum factor of safety equal to 5.6 for the pulley mounts. A detailed account of our analysis can be found on the page Ceiling-mounted bicycle lift structural analysis which includes both hand calculations and results from several finite element analyses.
| Part Number | Component | Safety Factor | |
|---|---|---|---|
| 1 | Ceiling Bracket | 12.9 | |
| 2 | U-Pulley Mount | 5.6 | |
| 3 | Pulley Wheel | 24.5 | |
| 4 | Pulley Rivet | 32.3 | |
| 8 | Locking Pulley Mount | 5.6 | |
| 12 | V-Bracket | 9.1 | |
| 13 | Hook | 7.4 | |
| 17 | Rope | 100+ | |
| 19 | Ceiling Screw | 58 |
Conclusion
The minimum factor of safety corresonds to a load of approximately 280 pounds. With the product's mechanical advantage of 4, this corresonds to a user input force of 70 pounds, which is definitely achievable, even though it's not in the realm of proper use of the lift. When we performed a physical test to try and confirm this result, the locking mechanism slipped at a load of roughly 250 pounds. So although we were unable to perform our destructive testing, we were able to uncover another clever subtlety to the product's design. By not allowing the locking mechanism to hold over 250 pounds, the ability to overload the bicycle lift to failure is virtually removed. We found this to be a very clever feature that allows for a very safe design even under improper use scenarios.
Design for Manufacture and Assembly (DFMA)
Summary
Our preliminary analysis concluded that overall this product was designed very well for manufacture and assembly. The majority of the product is made from the same material with similar manufacturing techniques, keeping the fabrication process simple and cost effective, without impairing product function. The incorporation of sub-assemblies, duplicate and symmetric parts allows for a straightforward and intuitive assembly process for the consumer.
There are a few areas in which minor design changes could result in a better product from the manufacturing and assembly viewpoint. There is a reasonable amount of assembly that occurs prior to packaging by your company. Some alternative designs might turn out to be a feasible solution to this problem and reduce the large amount of fasteners in the process. For example, combining a V-bracket(12) and two hooks(13) into a single part would eliminate 12 fastener components and remove 4 assembly steps. Another hook design change is to eliminate the process of adding it's coating, since it doesn't have a very critical role in the product's performance. Lastly, the two clutch parts(9,10) of the latching mechanism appear to be over-designed with relatively thick sheet metal for the loads they encounter. Our structural analysis is still in the process of possibly confirming this.
- Please refer to the page DFMA for a more detailed description of tools, methods and proccesses used in analyzing a product in terms of design for manufacture and assembly.
Manufacture
Many manufacturing cost and complexity considerations were incorporated into the design decisions of this bicycle lift, as can be seen by looking at the final design of this product. The total part count was kept very low, requiring only 19 unique parts (66 total) to perform the function of lifting and storing a bicycle. Every part plays a vital role in the operation of the product which is even more impressive considering that a lot of the parts appear to be standard parts, which are used in other products as well. The ceiling bracket(1), v-bracket(12) and pulley mounts(2,8) all have features that serve no purpose in this product, however these features don’t jeopardize the performance of their respective parts to any considerable degree. Not only are a lot of standard parts used, but 14 of the 19 unique parts are used more than once in this product. This utilization of duplicate and standard parts while keeping the total part count low is an excellent example of reducing manufacturing cost, since new tooling, suppliers and everything else that comes with the incorporation of a new part, is kept to a minimum.
The design decision with the biggest impact on overall product cost most likely is the choice of manufacturing method for the 10 unique non-fastener parts. 8 of these unique parts (16 total) are made from stamped steel sheet metal which is then bent to form the various somewhat complex shapes in a very cost effective manner. Making these high volume parts in this manner is very cost effective since it eliminates the need for holes to be drilled or for certain surfaces to be machined. Although stamping does result in some material waste, it appears as though this waste was minimized by the design of these stampings to be easily nested. Also, a lot of this material isn’t completely wasted, since it can be gather and then sold, or melted and used to create more stock.
5 of the 8 unique stamped parts are made from 2mm steel sheet, which also helps reduce cost by minimizing the variety of sheet metal that needs to be purchased. The other 3 require 2 different sheet metal thicknesses due to their required structural capabilities. The overall product design most likely could have been altered to allow for all components to be made from sheet metal with the same thickness, however this benefit would probably not outweigh the cost that these somewhat radical design changes would incur. Just the fact that 15 of the 19 unique parts are made from the same material (steel) is evidence of design decisions to communize material usage.
The other major part that shows evidence of involved manufacturing considerations is the pulley wheel(3) that appears 5 times in this product. This wheel is made from injection molded plastic, however the seam patterns suggest that it not made via the somewhat standard two-half mold, but rather a four-part mold that allows for the wheels complex geometries. Although it’s safe to assume that tooling for such a mold is more expensive, it allows for the pulley wheel to be made with over 30% less material. So although the initial capital cost is greater with this decision, the decreased material cost in the long run with large production volume will result in lower total cost.
In addition to these major categories of design decisions, there were various other considerations evident from the design of this product. Very near net shapes resulted from the previously described stamping process, thus resulting in very little, if any, machining. The two hooks(13) are the only stamped parts with evidence of post stamping work done on them, with both being dipped in some type of polymer residue to give the hooks a higher friction surface finish. The use of separate fasteners was minimized, with 19 screws used from only 3 unique parts. Tapers and contours were also kept somewhat to a minimum with only a handful of parts having non-rectangular shapes, thus reducing the manufacturing complexity. Lastly, the overall minimization of manufacturing complexity resulting from the many design decisions of this product allow for a design that requires relatively simple process capabilities.
Assembly
Design decisions for assembly are especially important for this product, since the majority of assembly is performed by the consumer. One way in which this design accomplishes that is via a base component from which other parts can be located. The two ceiling brackets(1) serve as the beginning of the assembly sequence and serve as a reference point from which the rest of the product can be viewed from. This design also allows for a very convenient assembly sequence, since everything can be mounted to the ceiling brackets and then these brackets can be attached to the ceiling in the final step. This allows for the majority of the assembly to be done in a convenient workspace as opposed to attaching the brackets to the ceiling first and then doing the rest of the assembly from there. This along with the overall product design makes for very high component accessibility, where most of the assembly is done in the open with minimal restrictions. There are some parts and sub-assemblies with less than desirable accessibility, however these parts are assembled prior to packaging with permanent fasteners, so the consumer doesn’t have to deal with them. Although, this accessibility could be improved, the resulting design changes would most likely result in a decrease of various other DFX’s, thus negating the benefit.
The minimization of total parts and use of duplicate parts as discussed previously from the fabrication point of view also aid in assembly, since less steps need to be performed and for the user, it’s easier to located a certain part since there are less unique parts to sift through. Along these lines, the incorporation of distinct sub-assemblies creates a more modular design adding to the overall ease of assembling the product. Many of the sub-assemblies are also duplicates, further increasing the ease of assembly. Parts such as the ceiling brackets(1), pulley mounts(2,8) and wall rope tie(18) are designed with clear symmetry, eliminating probable areas in which the consumer could incorrectly assemble a part. Although the number of unique fasters is low, the total number is somewhat high, when compared to the total number of parts in this product. This does increase assembly complexity and time, however for a product of this nature, this is somewhat difficult to avoid. Lastly, the product and its parts overall are very rigid with the rope being the only relatively flexible part. This along with all the other design decisions for assembly make the process of putting this bicycle lift together a much simpler task.
Design for Environment (DFE)
Several aspects of DFMA for our product also have beneficial intent to lessen the impact on the world's environment. Overall energy use is lowered by minimizing the total number of parts and maximizing repeated parts. Many of the parts in our product were manufactured using a stamping process. Stamping creates a relatively low amount of waste compared to other manufacturing processes and the waste from stamping can typically be recycled. Energy use in assembly of the product is also minimized by using repeated subassemblies and by leaving a majority of the assembly to be done by the user. Our product uses minimal packaging that would contribute to waste in landfills. The robust design of each component allows it to be packaged without any form of padding. They are simply wrapped in plastic bags inside the box. These bags are made of low density polyethylene plastic, which has a recycling code of 4. Leaving assembly to the user allows the parts to be packaged in a small box, which in turn allows more of the products to be carried per shipment and reduces environmental impact from transportation. Most of the parts are made of steel, which typically is a recycled material. Overall, the product is very well designed to reduce its impact on the environment. The only recommendation that we suggest is using recycled cardboard for the box that the product is packaged in.
- Please refer to the page DFE for a more detailed description of design for environment procedures and proccesses for analyzing a product.
Failure Mode and Effects Analysis (FMEA)
The purpose of this analysis is to identify ways the bicycle lift can fail and then determine causes for this failure as well as any effects it will have on the customer. This investigation will yield a risk priority number (RPN) for each failure mode allowing us to determine which modes require corrective actions, and how we suggest you go about implementing these corrections. The table on the page Ceiling-mounted bicycle lift FMEA consists of our detailed Failure Mode and Effects Analysis, however a brief summary is provided below outlining the key results of the study along with our suggestions for mitigating the risks that we consider to be unacceptable.
- Please refer to the page FMEA for a more detailed description of the purpose and format of a failure mode and effects analysis.
Overall this is a very safe product, putting the consumer at hardly any appreciable level of risk thanks to it's robust design and intuitive assembly and operation. There was however one component, the locking mechanism, that was the cause of two distinct failure modes with fairly high RPN's. If the user is standing on the side of the locking mechanism that the bike is on (not the normal standing position) and were to let the rope go, the locking mechanism would not be able to perform it's function of catching the rope and stopping the bike from free falling. The cause of this problem comes from the range of rotational motion of the clutches(9,10) on the locking mechanism. If this range was reduced by roughly 2 to 3 degrees, this entire failure mode could be eliminated. This could be accomplished with a small change to the design of the locking pulley mount(8), in which the bottom tab was made a few millimeters wider. This would not affect the current 8 degree rope angle needed for immediate engagement of the locking mechanism under proper usage. The repeated testing of this component ultimately led us to discover the second most severe failure mode of our analysis.
The components of the locking mechanism that pinch the rope, thus holding it in place, make contact with the rope over a relatively small area. Thus, repeated latch engagement is fairly harsh on the rope and after just several weeks of normal use, obvious signs of rope deterioration become apparent. The rope supplied with this product is sufficiently robust from a tensile load capacity standpoint. Replacing it with a rope made from a less standard material that has better surface wear resistance, although would be simple, we feel as though it would not make fiscal sense. Instead we propose a design change be made to the clutches(9,10) that engage the rope, in which they are given a more blunt surface that makes contact with the rope over a larger area, away from any sharp edges. This will not necessitate any other design changes to accomodate for this modification, which is why we believe this course of action to be the most cost effective change that solves the problem at hand.
Group Members: Jeff Polack, Kevin Pruzinsky, Davey Quinn, Russel Verbofsky
